Rate adaptation in a wireless communication system

ABSTRACT

Disclosed is a method of data rate adaptation based on channel conditions. In the present invention, data is initially transmitted at a first data rate based on a measured first channel condition and, if a NACK is received, the data is retransmitted. The data retransmitted is at a rate which is based on the condition of the channel during or before the transmission of the NACK. The data retransmission rate can also be based on the actual channel condition at the time of the first transmission plus the condition of the channel before or during the transmission of the NACK.

CROSS REFERENCE TO RELATED APPLICATION

Related subject matter is disclosed in the following applicationassigned to the same assignee hereof: U.S. patent application entitled“Sub-Packet Adaptation In A Wireless Communication System”, Ser. No.09/725,393, filed Nov. 29, 2000.

FIELD OF THE INVENTION

The present invention relates generally to wireless communicationsystems and, in particular, to rate adaptation of data transmission in awireless communication systems.

BACKGROUND OF THE RELATED ART

In the well-known Data Only Evolution of third generation CDMA basedwireless communication systems, hereinafter referred to as 3G-1xEVDO,voice and data services are provided using separate frequency carriers.That is, the voice and data signals are transmitted over separateforward links defined by different frequency carriers. Data istransmitted over a time multiplexed frequency carrier at fixed datatransmit powers but at variable data rates. Specifically, measuredSignal to Interference Ratio (SIR) at a receiver of a pilot signaltransmitted by a base station is used to determine a data rate which canbe supported by the receiver. Typically, the determined data ratecorresponds to a maximum data rate at which a minimum level of qualityof service can be achieved at the receiver. Higher measured SIRtranslates into higher data rates, wherein higher data rates involvehigher order modulation and weaker coding than lower data rates. Forexample, if measured SIR at the receiver is 12 dB and −2 dB at twodifferent receivers, then the data rates may be 2.4 Mb/s and 38.4 Kb/sat each of the respective receivers.

To improve system throughput, 3G-1x EVDO allows the receiver with themost favorable channel conditions, i.e., highest measured SIR, andthereby the highest associated data rate, to transmit ahead of receiverswith comparatively less favorable channel conditions. 3G-1xEVDO utilizesa fast rate adaptation mechanism whereby the receiver, for every timeslot, measures SIR, calculates a data rate using the measured SIR andreports the calculated data rate to the base station. Calculated datarates from multiple receivers are used by the base station to schedulewhen data transmission is to occur for a particular receiver.

Data transmission from the base station to a particular receiver occurswhen that receiver reports the highest calculated data rate to the basestation. The following protocol is utilized in data transmissions. Thebase station transmits data to the receiver in time slot n at thecalculated data rate. The receiver receives the data transmission andresponds with an ACK/NACK message indicating to the base station whetherthe data transmission was successfully received, i.e., no errors, by thereceiver. Specifically, if the data transmission is successfullyreceived, the receiver responds with an acknowledgement or ACK.Otherwise the receiver responds with a negative acknowledgement or NACK.The ACK/NACK message is received by base station in time slot n+j,wherein j is some known time offset. Thus, the base station candetermine that an ACK/NACK message was transmitted from a receiver towhich data was transmitted j time slots prior to receipt of the ACK/NACKmessage.

If an ACK was received, the base station knows that the datatransmission to the associated receiver was successful. If a NACK wasreceived, the base station knows that the data transmission to theassociated receiver was unsuccessful. In response to the NACK, the basestation re-transmits, at the same data rate, the same data which wasearlier transmitted. Note that the term “re-transmits the same data”should be understood to describe a retransmission of the data that mayor may not be identical to the data it is being compared to, i.e., datatransmitted in a previous transmission, so long as the data of theretransmission may be soft combined with the data to which it is beingcompared. The re-transmitted data is received by the receiver in timeslot n+j+k, wherein k is some known time offset.

This prior art protocol disadvantageously utilizes the data rate of theinitial transmission for re-transmissions even if the channel conditionsmay have changed for the associated receiver. Specifically, if thechannel conditions degraded between the time of the initial transmissionand the re-transmission, the re-transmission will likely suffer a higherframe error rate (FER) than the initial transmission, thereby sufferinga degradation in transmission quality. Or if the channel conditionsimproved, then channel resources are being inefficiently utilized sincea higher data rate could had been used for the re-transmission.

SUMMARY OF THE PRESENT INVENTION

The present invention is a method of data rate adaptation based onchannel conditions. In the present invention, data is initiallytransmitted at a first data rate based on a measured first channelcondition and, if a NACK is received, the data is retransmitted. Thedata retransmitted is at a rate which is based on the condition of thechannel during or before the transmission of the NACK. The dataretransmission rate can also be based on the actual channel condition atthe time of the first transmission plus the condition of the channelbefore or during the transmission of the NACK.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, aspects, and advantages of the present invention willbecome better understood with regard to the following description,appended claims, and accompanying drawings where:

FIG. 1 depicts a flowchart illustrating the data rate adaptationtechnique in accordance with one embodiment of the present invention;

FIG. 2 depicts a flowchart illustrating a manner of varying the size ofthe sub-packets, the modulation scheme and number of time slots overwhich the sub-packets are transmitted in accordance with one embodimentof the present invention;

FIG. 3 depicts a flowchart illustrating a manner of varying the size ofthe sub-packets, the modulation scheme and number of time slots overwhich the sub-packets are transmitted in accordance with one embodimentof the present invention; and

FIG. 4 is an example of rate adaptation with three slots rate feedbackdelay.

DETAILED DESCRIPTION

The present invention is a method of data rate adaptation based onchannel conditions. FIG. 1 depicts a flowchart 100 illustrating the datarate adaptation technique in accordance with one embodiment of thepresent invention. In step 110, a base station or transmitting equipmentreceives rate indication messages from a plurality of receivers to whichdata transmissions are intended, wherein a rate indication message maybe a channel condition measurement at a receiver or a data ratecalculated based on a channel condition measurement at a receiver. Instep 115, the base station selects a receiver at which to transmit data,wherein the selected receiver preferably is associated with the highestdata rate. In step 120, the base station transmits a sub-packet of datato the selected receiver at the data rate indicated by the associatedrate indication message.

In another embodiment, the sub-packet transmitted in step 120 may betransmitted at a data rate higher than the data rate indicated in therate indication message. The reason for doing this is to decrease theamount of time slots over which the sub-packets are to be transmitted instep 120. Although the transmission quality may degrade because of theincreased data rate, Hybrid ARQ may be used to soft combine thesub-packets transmitted in step 140 with the sub-packets transmitted instep 120. Under certain conditions, e.g. at lower data rates, when usingHybrid ARQ (soft combining) throughput efficiency of the channel can beimproved through the “aggressive” use of the channel, i.e., transmittingat higher data rates than indicated by the receiver.

The data rate at which the encoder sub-packets are transmitted may benegotiated between the base station and receiver anytime prior to theactual transmission of the encoder sub-packets. For example, thereceiver transmits a rate indication message to the base stationindicating a data rate of 19.2 Kb/s. The base station wants to beaggressive with the data transmission by using a data rate of 76.8 Kb/sto transmit an encoder sub-packet to the receiver. Accordingly, the basestation transmits a new rate message to the receiver indicating the newdata rate at which the base station will be transmitting the encodersub-packet to the receiver, wherein the new data rate indicated may ormay not be the same as the data rate indicated in the data rate message.Upon receipt of the new rate message, the receiver would know the datarate to use in decoding the encoder sub-packet.

The new data rate is based on the data rate message and the size of theencoder packet. For larger size encoder packets, it is desirable to setthe new data rate as a higher multiple, e.g., four times, of the datarate indicated in the data rate message in order to reduce the number oftime slots utilized in the transmission and to promote schedulingflexibility. By contrast, for smaller size encoder packets, it isdesirable to set the new data rate as a lower multiple, e.g., one times,of the data rate indicated in the data rate message in order to utilizethe channel more efficiently.

Table I depicts an example lookup table which may be used in selecting anew data rate based on the data rate indicated by the receiver and thesize of the encoder packet. For example, suppose the data rate messageindicates a data rate of 38.4 Kb/s and the encoder packet is 1,536 bits.The new rate message would then indicate a new data rate of 153.6 Kb/s.

TABLE I Data Rate Data Rates For Data Rates For Data Rates For DataRates For Indicated In Data 7,680 Bit 3,072 Bit 1,536 Bit 768 BitEncoder Rate Message Encoder Packet Encoder Packet Encoder Packet PacketKb/s Kb/s Kb/s Kb/s Kb/s 9.6 38.4 38.4 38.4 38.4 19.2 76.8 76.8 76.876.8 38.4 153.6 153.6 153.6 153.6 76.8 307.2 307.2 307.2 307.2 153.6614.4 614.4 614.4 614.4 307.6 877.7 819.2 614.4 614.4 614.4 1228.81228.8 1228.8 614.4 819.2 1536.0 1228.8 1228.8 614.4 1228.8 2048.02457.6 1228.8 614.4 1536.0 3072.0 2457.6 1228.8 614.4 2048.0 3072.02457.6 1228.8 614.4 2457.6 3072.0 2457.6 1228.8 614.4

In step 125, the base station receives an ACK/NACK message from theselected receiver. If the message is an ACK, in step 130, flowchart 100returns to step 110. If the message is a NACK, in step 135, the basestation receives from the selected receiver another rate (rateadaptation) indication message. Additionally, when a NACK is transmittedby the receiver, the receiver stores in memory the received data whichwas transmitted in step 120 such that it may later be soft combined witha re-transmission of the same data.

An example of rate adaptation is shown in FIG. 4. The transmission rateused in slot n is based on channel condition (quality) measurements inslot (n−3) assuming three slots feedback delay. With Hybrid ARQ, if aframe is received in error, the receiver stores the frame and sends backa NACK to the transmitter. The transmitter performs a retransmission andthe two transmissions can be combined and the frame can be decoded withhigher success probability. The number of transmissions/retransmissionacross which the hybrid ARQ operation is done can be a larger numbergreater than 2.

With adaptive Hybrid ARQ, the retransmissions can be performed at adifferent rate compared to the original transmission. The rate at thetime of retransmission is based on the most recent channel conditioninformation received from the receiver. The ACK/NACK for transmission inslot n is received in slot (n+3) where 3 is the ACK/NACK feedback delay.

If the message is a NACK, in step 135, the base station receives fromthe selected receiver another rate indication message which is based onthe delayed channel condition information that is due to feedback delayin addition to the actual channel condition at the time of the previoustransmission. Stated differently, the rate of retransmission is based onestimating the channel condition at the time of transmission of theNACK. The rate of retransmission can also be based on estimating twoconditions, one being channel condition at the time of transmission ofthe NACK; and the other estimate being the actual channel condition atthe time of the transmission of the data. The estimates enable thetransmitter to make an estimate about the quality of the previouslyreceived information and the amount of redundant information needed tosuccessfully decode the received data.

In step 140, the base station re-transmits the sub-packet of data to theselected receiver at the data rate indicated in the second rateindication message received in step 135.

In one embodiment, the sub-packet of data transmitted in steps 120 and140 are of the same size but the number of time slots over which thesub-packets are transmitted or modulation scheme may vary if the datarates in steps 120 and 140 are different. In another embodiment, suchsub-packet are of different sizes if Hybrid ARQ are used to soft combinethe sub-packets transmitted in steps 120 and 140.

In an alternate embodiment, regardless of whether the ACK/NACK messagetransmitted by the selected receiver is an ACK or a NACK, flowchart 100returns to step 110 from step 125. In this embodiment, there-transmission to the originally selected receiver would not occuruntil the selected receiver is the receiver with the highest associateddata rate.

In a preferred embodiment, the manner in which sub-packets aretransmitted in steps 120 and 140 allows for Hybrid ARQ at different datarates. This embodiment is achieved by varying the size of thesub-packets, the modulation scheme and number of time slots over whichthe sub-packets are transmitted. FIG. 2 depicts a flowchart 200illustrating a manner of varying the size of the sub-packets, themodulation scheme and number of time slots over which the sub-packetsare transmitted in accordance with one embodiment of the presentinvention. In step 210, at the connection set-up to a new receiver, orthrough other broadcast means, the base station indicates to thereceiver the data transmission rate that will be used by the basestation corresponding to a rate indication message from the receiver andeach of the encoder packet sizes (as shown in Table 1). Alternatively,the base station transmits a new rate message to the selected receiverindicating the new data rate at which the base station intends totransmit data to the selected receiver. In another embodiment, the newrate message may be included in the header information or along with theencoder packet size indication. In step 215, an encoder packet isprocessed into a specific size encoder sub-packet, wherein the encoderpacket is a block of information intended for the receiver and theencoder sub-packet is a representation of the encoder packet which istransmitted to the receiver. Specifically, the encoder packet is channelcoded and subsequently punctured and/or repeated to obtain a sub-packet.The size of the sub-packet being dependent on the data rate at which thesub-packet is to be transmitted and the size of the encoder packet.

FIG. 3 depicts an example 30 of a sub-packet formation scheme inaccordance with this embodiment of the present invention. An encoderpacket comprising of 3,072 bits is turbo coded at ⅕ rate into 15,360bits. Note that, in this example, a same channel coder is used tochannel code the encoder packet regardless of the size of thesub-packet. The channel coded encoder packet, i.e., 15,360 bits, thenundergoes different puncturing and/or repetition techniques to obtainfour different size encoder sub-packets, wherein the original encoderpacket may be derived from each of the encoder sub-packets.Specifically, the channel coded encoder packet is punctured and/orrepeated to produce two 13,824 bit encoder sub-packets, one 24,576 bitencoder sub-packet, two 12,288 bit encoder sub-packets and/or three6,144 bit encoder sub-packets. The two 13,824 bit encoder sub-packet mayor may not be identical to each other. Likewise for the two 12,288 bitencoder sub-packets and three 6,144 bit encoder sub-packets. Each of theencoder sub-packets may be soft combined with each other.

Note that each of the encoder sub-packets are associated with differentdata rates. That is, the two 13,824 bit encoder sub-packets areassociated with a data rate of 819.2 Kb/s; the 24,576 bit encodersub-packet is associated with data rates of 38.4 Kb/s, 76.8 Kb/s, 153.6Kb/s and 307.2 Kb/s; the two 12,288 bit encoder sub-packets areassociated with data rates of 614.4 Kb/s and 1288.8 Kb/s; and the three6,144 bit encoder sub-packets are associated with a data rate of 2457.6Kb/s. Thus, if the data rate at which the sub-packet was to betransmitted was 153.6 Kb/s, the sub-packet size would be 24,576 bits.Note that there exists a single sub-packet format for a given data rateand encoder packet size. Although FIG. 3 depicts all eight differentsub-packets being simultaneously produced, all eight of the encodersub-packets need not be produced at the same time.

Returning to FIG. 2, in step 220, an encoder packet size identifier isadded to the encoder sub-packet, wherein the encoder packet sizeidentifier indicates the size of the packet from which the encodersub-packet was derived. Based on the encoder packet size identifier andthe transmission data rate, the receiver can determine the format of thesub-packet such that the receiver can correctly soft combine and jointlydecode the associated encoder sub-packet with a re-transmission or aprior transmission of an encoder sub-packet derived from the sameencoder packet (although the latter sub-packet may be in a differentformat). Recall that there exists a single sub-packet format for a givendata rate and encoder packet size. The data rate is known to thereceiver based on one of many alternate embodiments discussed above. Thetransmission data rate is mapped from the rate indication message fromthe receiver, either based on a mapping that is indicated to thereceiver at connection set-up, or on a broadcast channel. Otherwise, thetransmission data rate is transmitted in a message or in data headerinformation to the receiver.

In another embodiment, whether or not there exists a single sub-packetformat for a given data rate and encoder packet size, an encodersub-packet format identifier may be added to the encoder sub-packet inlieu of, or in conjunction with, the encoder sub-packet size identifier.The encoder sub-packet format identifier indicating a format of theassociated encoder sub-packet such that the receiver knows how to derivethe encoder packet from the encoder sub-packet.

In step 225, the encoder sub-packet is modulated and transmitted to thereceiver over one or more time slots. The type of modulation scheme usedto modulate the encoder sub-packet depends on the new data rate. TableII depicts an example lookup table which may be used in selecting amodulation scheme based on the new data rate. As can be seen, highermodulations (with more bits per symbol) are required to achieve thehigher data rates. For example, if the new data rate is 307.2 Kb/s, thenthe modulation scheme used to transmit the encoder sub-packet would beQPSK.

TABLE II New Data Rate Modulation Scheme 9.6 QPSK 19.2 QPSK 38.4 QPSK76.8 QPSK 153.6 QPSK 307.2 QPSK 614.4 QPSK 819.2 8-PSK 1228.8QPSK/16-QAM 1536.0 16-QAM 2048.0 16-QAM 2457.6 16-QAM 3072.2 16-QAM

The number of time slots used in the transmission of the encodersub-packet depends on the new data rate and the size of the encoderpacket (or encoder sub-packet). Table III depicts an example lookuptable which may used in determining the number of time slots requiredfor transmitting a particular size encoder packet at the new data rate.

TABLE III 7,680 Bit 3,072 Bit 1,536 Bit 768 Bit Encoder Encoder PacketEncoder Packet Encoder Packet Packet Data Time Data Time Data Time DataTime Rate Slots Rate Slots Rate Slots Rate Slots 38.4 160 38.4 64 38.432 38.4 16 76.8 80 76.8 32 76.8 16 76.8 8 153.6 40 153.6 16 153.6 8153.6 4 307.2 20 307.2 8 307.2 4 307.2 2 614.4 10 614.4 4 614.4 2 614.41 877.7 7 819.2 3 614.4 2 614.4 1 1228.8 5 1228.8 2 1228.8 1 614.4 11536.0 4 1228.8 2 1228.8 1 614.4 1 2048.0 3 2457.6 1 1228.8 1 614.4 13072.0 2 2457.6 1 1228.8 1 614.4 1 3072.0 2 2457.6 1 1228.8 1 614.4 13072.0 2 2457.6 1 1228.8 1 614.4 1

Although the present invention has been described in considerable detailwith reference to certain embodiments, other versions are possible. Forexample, the present invention is also applicable to encoder packetswhich are not 3,072 bits in size; the encoder sub-packet sizes may vary;the data rate at which particular encoder sub-packets may vary; etc.Therefore, the spirit and scope of the present invention should not belimited to the description of the embodiments contained herein.

We claim:
 1. A method of communicating with a receiver comprising thesteps of: transmitting data to the receiver at a first data rate that isdetermined at least in part based on first received channel conditioninformation; and in response to an indication that the transmitted datawas not successfully received, retransmitting the data to the receiverat a second data rate that is determined at least in part based on thefirst received channel condition information and on second receivedchannel condition information that is received subsequent to the firstchannel condition information.
 2. The method of claim 1 wherein thesecond data rate is determined in part based on an estimate of the dataredundancy needed for successfully decoding the data.
 3. The method ofclaim 1 wherein the transmitted data and the retransmitted data areidentical.
 4. The method of claim 1 wherein the retransmitted data maybe soft combined with the transmitted data.
 5. The method of claim 1wherein the first received channel condition information is a first rateindication message and the first data rate is a rate that is higher thana data rate indicated in the first rate indication message.
 6. Themethod of claim 1 wherein the second received channel conditioninformation is a second rate indication message and the second data rateis a rate that is higher than a data rate indicated in the second rateindication message.
 7. The method of claim 1 wherein the indication thatthe transmitted data was not successfully received is a NACK.